T3D Re: the eye's in-focus surface in object space

[john bercovitz <bercov@xxxxxxxxxxx>]

I just "got" something you said, BobH.  I'm slow, but I get there... 
sometimes.

> Don't forget our eyes are focused on a curved surface...
[...]
> so we can use simple lenses whose plane of focus test to be a dished one. 

I don't think so and here's why.  When you look at different places on 
the transparency, you will have to refocus since the flat transparency 
will not fit the curved field of the lens.  In designing an eyepiece, 
you try to keep the field as flat as possible, a very tough job, but if 
anything, you make the field curvature such that the eye is relaxed when 
viewing near the center of the image and has to accommodate when viewing 
the corners.

To see what a simple lens does, I recommend using Joel's Ocular Ray 
Tracer because it's so easy.  It's at 
http://www.frii.com/~rkymtmem/ort/index.html

For a simple lens, choose PCX (plano-convex).  Just picking some numbers 
off the top of my head, use 34 mm from the lens surface to the center of 
rotation of the eye, 35 mm for a lens diameter, and 25 mm for a back radius.  
Then a lens thickness of 8 mm should leave you some edge thickness.  Use a 
Realist format and use a lighter crown like BK 7. You'll see a whole lot of 
field curvature, I guarantee it, but this is like a lens used in cheap 
viewers.  For better results, pull some cemented doublet data from 
the Edmund catalog.

John 


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